docs: def _from_directory(self, dirname, prefix=""):
          """Add dataset from files in a directory.
  
          Parameters
          ----------
          dirname : string
              Directory name.
          prefix : string
              Prefix.
          """
          ps = [os.path.join(here, dirname, p) for p in os.listdir(os.path.join(here, dirname))]
          n = prefix + wt_kit.string2identifier(os.path.basename(dirname))
          setattr(self, n, ps)